1. Field of the Invention
The present invention relates to a voltage measuring device for use in measuring the voltage of each battery cell in a battery cell array in which a predetermined number of battery cells are connected in series.
2. Description of the Related Art
Lithium ion batteries, which have a higher output voltage, higher energy density, and higher efficiency than general secondary batteries, are often used as batteries to be installed in hybrid electric vehicles (HEV) and electric vehicles (EV). It is difficult to control charging and discharging of lithium ion batteries, and lithium ion batteries have a risk of explosion and firing. Therefore, in a case where a lithium ion battery is used as an in-vehicle battery, it is particularly important to manage the battery voltage.
There is a known configuration of a voltage measuring device for use in measuring the voltage of such a battery. The known configuration is such that, for battery cells connected in series, the voltage between both ends of each battery cell is measured sequentially (see Japanese Laid-Open Patent Application Publication No. 2009-174961 and Japanese Laid-Open Patent Application Publication No. 2010-60435, for example). FIG. 9 is a circuit diagram showing a schematic configuration of a conventional voltage measuring device. As shown in FIG. 9, a plurality of (n) battery cells Cj (j=1, . . . , n) are connected in series in a battery cell array 101, and in order to measure the voltage of each battery cell Cj, a conventional voltage measuring device 110 includes a multiplexer 102 and a signal processor 103. Voltages VCi (i=0, 1, . . . , n) at connecting ends of the plurality of battery cells Cj are inputted to the multiplexer 102, and the multiplexer 102 outputs one of the voltages. Based on the output from the multiplexer 102, the signal processor 103 performs operational processing on the voltage of each battery cell Cj. The multiplexer 102 includes pairs of switches SWAi and SWBi. Each one of the pairs of switches SWAi and SWBi is provided corresponding to a respective one of a plurality of first input terminals Ti (i=0, . . . , n). The voltages at the connecting ends of the plurality of battery cells Cj are inputted to the plurality of first input terminals Ti. When one switch SWAi is turned on, the voltage at one end of a corresponding battery cell Cj is connected to one of a pair of output terminals TO1 and TO2 (specifically, connected to the output terminal TO1 at the positive phase side). When the other switch SWBi is turned on, the voltage at the one end of the corresponding battery cell Cj is connected to the other one of the pair of output terminals TO1 and TO2 (specifically, connected to the output terminal TO2 at the negative phase side). The signal processor 103 includes: 1) a pair of capacitors 131A and 131B whose one ends are connected to the pair of output terminals TO1 and TO2 of the multiplexer 102, respectively, wherein, the pair of capacitors 131A and 131B are charged in accordance with voltages outputted from the respective output terminals TO1 and TO2; 2) a differential amplifier 130 including a pair of input terminals, which are connected to the other ends of the pair of capacitors 131A and 131B, respectively; 3) a pair of capacitors 132A and 132B connected between the input terminals and output terminals of the differential amplifier 130; and 4) a pair of switches 133A and 133B connected parallel to the pair of capacitors 132A and 132B. That is, the signal processor 103 is configured as a so-called level shifter circuit.
As described above, by means of the multiplexer 102, nodes at both ends of a battery cell Cj are selectively connected to the signal processor 103 via switches SWAi and SWBi corresponding to the nodes, and thereby the voltage of the battery cell Cj is measured. This process is performed on all of the battery cells Cj sequentially, and thereby voltage data of each battery cell Cj is collected. Based on the collected voltage data, control of the battery is performed.